1. Field of the Invention
The present invention generally relates to charged particle beam systems and, more particularly, to control of beam current in electron beam projection lithography tools.
2. Description of the Prior Art
Lithographic processes are utilized in the manufacture of many diverse types of devices, particularly when very small areas must be selectively defined and/or operated upon, as in semiconductor integrated circuit manufacture. At least one lithographic process is invariably required for initial definition of locations and basic dimensions of devices such as transistors and capacitors in integrated circuits.
Lithographic processes currently used for integrated circuit manufacture involve the selective exposure of areas of a resist coated on a surface. In general, depending on whether the resist is of a positive or negative type, subsequent development will selectively remove either the exposed or unexposed areas leaving other areas substantially unaffected. In the past, radiant energy has been the resist exposure medium of choice.
However, modern integrated circuit designs require feature sizes smaller than can be resolved using even very short wavelengths of light in the deep ultra-violet range with sophisticated devices such as phase shift masks, optical proximity or off-axis illumination. Exposure of the resist with charged particle beams is required to obtain smaller feature sizes which are becoming increasingly common in current integrated circuit designs. Electron beams are generally preferred for charged particle beam exposures since, among other relative advantages, the low mass of electrons allows control of the beam with relatively less power.
So-called probe-forming systems form a single well-focused spot at the target surface for exposure of the resist. This spot can be either scanned across selected portions of the surface or discrete spot exposures made in a step-and-repeat fashion. However, a single exposure is generally limited to a few hundred pixels, at most, while the full pattern required for a single integrated circuit chip may include hundreds of millions of pixels or more. Therefore, the throughput of probe-forming exposure tools is too low to be economically feasible for high density, large scale integrated circuits even though exposures can be made at relatively high speed.
To obtain acceptable levels of throughput, electron beam projection lithography has been recently developed. Projection lithography projects a pattern (which may contain several millions of pixels) from a subfield of a reticle which selectively intercepts or scatters portions of a relatively wide beam of charged particles. The pattern can be demagnified by the charged particle optics of the tool so that the pattern at the target is much smaller than the subfield pattern formed in the reticle. The subfield patterns formed in the reticle are stitched together in sequential exposures to form the overall circuit pattern of the complete integrated circuit design.
While the charged particle beam column of electron beam projection tools utilizes many elements which are more-or-less analogous to certain elements found in optical systems (and hence are often referred to as electron-optical elements or simply as lenses, stigmators and the like), many of the interrelationships among operational parameters of a charged particle beam system are often much more complex. For example, at a given beam current, aberrations are minimized and resolution is optimized only for a single value of numerical aperture (particle trajectory semi-angle) and vice-versa due to the contributions of stochastic Coulomb interactions, spherical aberration and chromatic aberration, each of which varies differently with changes in numerical aperture.
Thus, for any given numerical aperture, resolution varies with beam current as well as numerous other parameters of the particle beam column. For example, it is desirable to maintain the particle beam column as short as possible in order to limit the distance over which particles can interact, thus reducing the stochastic interactions among the electrons. However, reduction of beam current requires a corresponding increase in exposure time and small increases in individual subfield exposure time can have a major adverse impact on tool throughput in view of the number of subfield exposures that must be made for a single chip.
The variation of resolution with beam current presents a particularly intractable problem in charged particle beam projection systems which project a pattern established by a reticle subfield placed in the path of the charged particle (e.g. electron) beam. In practice, the defocus caused by the space-charge component of the electron interactions is corrected by refocusing the beam as the current is changed. However, the stochastic interactions are not correctable and vary as less than the first power of current.
To maximize throughput, it is expected that the placement of subfields in the reticle will match the pattern established on the integrated circuit by the designer. For integrated circuits containing memory (DRAM) cells, there is a substantially regular pattern of similar features and dimensions for many subfields. Therefore the transmission of the beam from respective subfields, and the resulting beam current of the patterned beam striking the wafer will be nearly identical. It is possible, on the other hand, that for other integrated circuits, the transmission from respective, and adjacent subfields, might differ substantially. To compensate for the space-charge defocusing, the beam is refocused on a subfield-by-subfield basis. Variations in the resolution between features of differing transmission is caused by the stochastic Coulomb interactions as discussed previously. This can affect the critical dimension (C/D) uniformity, which is usually specified as about 10% of the beam resolution.
Alteration of the beam current above the reticle to compensate for differing reticle subfield transmissivities, while possible (although changing the illumination optics cannot currently be accomplished with adequate speed), presents other problems and is not considered to be suitable. Thermal loading on critical elements (e.g., shaping, contrast and blanking apertures) would change with illumination current which might lead to a distortion of these elements causing alignment or illumination changes and/or astigmatism of the beam. These possible effects could require correction in order to maintain high image fidelity on the wafer. Furthermore, dynamically changing the beam current above the reticle causes the current density of the beam to change, which affects the electron dose (microCoulombs/cm.sup.2) at the wafer. The exposure time would then have to change as a function of the beam current impinging on the reticle to keep the electron dose constant.
In general, operating parameters of charged particle beam systems may be approximately determined through simulation in order to consider the multiple interactions and variables of a complex charged particle beam column. However, final setting of operational parameters must be verified by testing of the actual performance of the charged particle beam tool. This process may be extremely expensive and time-consuming, particularly when hardware adjustments are required, such as alteration of numerical aperture.
Verification of the resolution of the beam over an entire integrated circuit pattern is especially difficult, primarily because many measurements of the C/D must be made. Furthermore, it is necessary to refocus the beam (to compensate for the space-charge component of the electron-electron interactions) depending on reticle subfield transmissivity. To assess the resolution as a function of current impinging on the wafer, one would have to deconvolve the measurements with the resist response. This might be of limited utility if there were a small range of subfield transmissivities on the reticle which would produce a limited range of beam currents.
Accordingly, there is a need for an arrangement for experimentally studying the resolution performance of charged-particle beam columns in response to changes of beam current. This is necessary to calibrate theoretical models, as well as optimize the numerical aperture. One unattractive solution would be to use one reticle subfield and alter the current impinging on the reticle, then refocus the beam and measure the resolution (either by a direct method or by writing a wafer), then change to beam current and so on. Unfortunately, when the beam current above the reticle is changed, there might be effects of charging, which might cause astigmatism, as mentioned above, and/or loss of resolution.
A more attractive alternative would be to keep the current impinging on the reticle constant and change the subfield transmissivity. However, prior to the present invention, it was only possible to change to a different subfield with a different discrete transmissivity and pattern which would therefore be limited to "sampling" of resolution at different beam currents and current density distributions. Such discrete smples may obscure important information, particularly in regard to complex system performance. Thus there is a need for an arrangement capable or producing continuously variable controllable beam current, by control of the transmissivity of a reticle subfield.